1. Field of the Invention
This invention relates to an improved catalyst and method for using the catalyst for hydrogenation in aqueous solution. More specifically but not by way of limitation, the invention relates to the production of tetrahydrofuran, .gamma.-butyrolactone, 1,4 butanediol and the like from a hydrogenatable precursor such as maleic acid, succinic acid or the like in aqueous solution in the presence of hydrogen and a catalyst consisting essentially of highly dispersed, reduced ruthenium and rhenium on a carbon support.
2. Description of the Related Art
Various methods and reaction systems have been proposed in the past for manufacturing tetrahydrofuran, .gamma.-butyrolactone and 1,4-butanediol by catalytic hydrogenation of maleic acid, maleic anhydride, succinic acid and/or related hydrogenatable precursors. Also, a variety of hydrogenation catalyst have been historically proposed for this purpose including various transition metals and combinations of transition metals deposited on various inert supports, all as generally known in the art. For example, U.S. Pat. Nos. 4,985,572 and 5,149,680 disclose and claim a process for hydrogenating a carboxylic acid or an anhydride thereof to corresponding alcohol and/or carboxylic acid ester using a catalyst composition comprising an alloy of a noble metal of Group VIII and one other metal. In a comparative test a ruthenium and rhenium supported on graphitized carbon is used as a catalyst in a plug flow single pass hydrogenation of acetic acid. Also, Japanese patent application publications (Kokai) 6-157490, 6-179667 and 6-157491 disclose methods of preparing tetrahydrofuran by catalytic hydrogenation of maleic anhydride or maleic acid under milder conditions in the presence of an acidic substance using as catalyst a rhenium compound and a Group VIII metal in the first two references and a ruthenium compound in the latter. Comparative examples in the first two references illustrated the use of a ruthenium and rhenium on carbon catalyst without the acidic substance being present. However as a general rule these proposed catalytic reactions are predominantly conducted in an organic solvent or organic reaction media and not in an aqueous solution phase. In fact, at least one prior publication suggests that water and succinic acid may be considered as inhibitors to the desired catalysis, see Bulletin of Japan Petroleum Institute, Volume 12, pages 89 to 96 (1970). One notable exception to the lack of aqueous phase catalytic hydrogenation is the use of a carbon supported catalyst comprising 0.5% to 10% palladium and about 1% to 10% rhenium by total weight of supported catalyst in an aqueous solution hydrogenation reaction wherein the palladium is present on the carbon support in the form of crystallites having an average size of about 10 nm to 25 nm and the rhenium is present in a highly dispersed phase having an average size less than about 2.5 nm as described in U.S. Pat. Nos. 4,550,185; 4,609,636; and 4,659,686. Also, in a recently laid-open Japanese Kokai, Sep. 24, 1993, the use of reduced ruthenium and tin on an activated carbon support is disclosed in aqueous phase catalytic hydrogenation. This reference teaches the use of any Group VIII noble metal including palladium and ruthenium in combination with either tin, rhenium or germanium. The reference distinguishes the claimed subject matter from the previous Pd, Re/Carbon aqueous phase hydrogenation system of the prior art by virtue of specifically claiming the use of a carrier of porous carbon having a BET surface area of at least 2,000 m.sup.2 /g; a concept and limitation that is not characteristic of the Pd, Re/Carbon system of the U.S. Pat. Nos. 4,550,185 and 4,609,636 patents nor is this limitation of any critical significance relative to the present invention.